1. Field of the invention
The present invention relates to mixed oxides of alumina and zirconia consisting essentially of spherical particles having submicronic size.
In particular it relates to mixed oxides of alumina and zirconia consisting essentially of spherical, non-aggregated particles, having controlled granulometric distribution.
2. The Prior Art
It is known that alumina, in its different crystalline forms, may be used in various ways, for instance, such as material for electronics, in the preparation of semiconductors substrates and in integrated circuits packing, as ceramic material for the preparation of cutting tools, or as catalyst support in heterogeneous catalysis.
Alumina-based sintered materials do not exhibit, however, very high mechanical strength, for instance, cutting tools made therefrom tend to lose their good characteristics during working.
Furthermore such materials have a low tensile strength as well.
It is well known that the low strength characteristics mostly come from the lack of hardness of the sintered material.
A significant improvement of the aforesaid properties is obtained by using alumina, in its different crystalline forms, in the form of monodispersed or polydispersed, non-agglomerate, spherical particles with narrow granulometric distribution, having a submicronic size or a size up to a few microns.
Several methods are known for obtaining hydrated alumina in the form of spherical particles, such as, for instance, the process described in "Aluminum Hydrous Oxide Sols--II--Preparation of Uniform Spherical Particles by Hydrolysis of Al-sec-Butoxide" P. L. Catone e E. Matijevic, Journal of Colloid Interface Science, Vol. 48, No 2, August 1974, page 291.
A process allowing to obtain hydrated alumina with a very high productivity was described in a preceding patent application of the Applicant. (Italian Patent Application No 19959 A/85).
The hydrated alumina thus obtained can be subjected to thermal treatments in order to obtain gamma-alumina, which retains the same morphologic and granulometric characteristics of the starting precursor. With regard to this, we can cite, for instance, the article of E. Matijevic et al. "Heterogeneous Atmospheric Chemistry Geophysical Monograph Series", vol. 26, pages 44-49 (1982).
Furthermore in another preceding patent application of the Applicant (Italian Patent Application No 19142 A/85), the possibility is described of obtaining alpha-alumina, or mixtures of crystalline phases comprising alpha-alumina with theta-, and/or eta-, and/or delta-alumina consisting essentially of spherical particles which retain the granulometric and morphologic characteristics of the starting precursor.
In fact, it is known that the morphology and the granulometric distribution exert a great influence on the mechanical, thermal, and electrical properties of the materials obtained from said aluminas, in the applications set forth hereinbefore.
Furthermore it was proved that the tensile strength of the alumina may be improved by dispersing zirconia particles in alumina itself. In the article "Preparation of Alumina-Zirconia Powders by Evaporative Decomposition of Solution," Comm. of the Amer. Ceram. Society, May 1984, pages C-92, by D. W. Sproson and G. L. Messing, the authors assert that the strength improvement is due to the presence of zirconia stabilized in the tetragonal phase.
In fact, it is known that zirconia can exist in different crystalline phases, the monoclinic phase up to about 1200.degree. C., the tetragonal phase from 1200.degree. to 2400.degree. C. and the cubic phase at temperatures over 2400.degree. C.
The improvement in the mechanical properties of alumina is obtained only if the zirconia distributed in the aluminum retains the tetragonal form at room temperature and if it does not convert into the monoclinic phase, as having subjected after being the material, based on aluminum oxide, to thermal treatment between 1200.degree. and 2400.degree. C. The alumina should stabilize the zirconia in the tetragonal phase and it should hinder its transition into the monoclinic phase, as the product, after being subjected to thermal sintering treatment is brought to room temperature.
Zirconia stabilization in the tetragonal phase may, however, be only obtained if the crystallite size of zirconia dispersed in alumina ranges within some critical values, usually between 0.2 and 0.6 .mu.m.
With regard to this we can cite, for instance, the article by E. P. Butler and A. H. Hener, "X-Ray Microanalysis of ZrO.sub.2 Particles in ZrO.sub.2 --Toughened Al.sub.2 O.sub.3 ". Communications of the Amer. Ceram. Soc., December 1982, page C-206, or issued European patent application EP 119,028.
The uniform distribution of the zirconia particles in the alumina is essential to obtain the described improvements in mechanical strength and hardness, since it is well known that if zirconia particles are distributed in an non-uniform way, dishomogeneities form in the product, which give rise to cracks during the sintering process. This phenomenon is described for instance in "Processing-Related Fracture Origins III, Differential Sintering of ZrO.sub.2, Agglomerates in Al.sub.2 O.sub.3 /ZrO.sub.2 Composite" by F. F. Lange and B. I. Davis, Journal Am. Ceram. Soc. 66 (6), 1983, page 407.
Alumina-zirconia mixed oxides so far known, having the described dispersion characteristics, are not in the form of spherical particles, monodispersed or polydispersed with a low polydispersion index, but they are in the form of agglomerated particles and above all they are lacking in sphericity.
Although the zirconia is uniformly distributed in alumina, the product shows under the electron microscope the lack of any geometric form and indeed it is present in the form of agglomerates.
Therefore in these mixed oxides all the property improvements mentioned hereinbefore are lost, namely the improvements due to sphericity, non-agglomeration and granulometric distribution, as set forth hereinbefore for alumina.
In fact, by using the methods known in the prior art for preparing zirconia -toughened alumina (ZTA), as described hereinbefore, although one works under conditions wherein alumina is obtained in the form of spherical particles by means of the cited processes, alumina-zirconia mixed oxides are obtained which do not present the morphologic and granulometric characteristics of the alumina as such.
Therefore the necessity arises to have mixed oxides of alumina and zirconia which combine the improvements of the strength characteristics of alumina, obtainable by means of zirconia stabilized in the tetragonal form, with these obtainable from alumina in the form of spherical particles and having a narrow granulometric distribution.